test: Avoid most situations involving failure to delete test directories

By forking a worker process and only deleting the test directory once it exits.

This way, if a test leaves files open, they'll get closed when the worker
exits, so avoiding failure to delete open files on Windows, and failure to
delete directories due to NFS lock files.

If a test leaves a git worker process running, the closed pipes should
cause the worker to exit too, also avoiding the problem there. The 10
second sleep ought to give plenty of time for such worker processes to
exit, although this is of course a race.

Finally, even if test directory fails to be deleted still,
it won't appear as if the last test in the test suite failed; the error
will be displayed at the very end.

This commit was supported by the NSF-funded DataLad project.

Utility/Misc.hs

@@ -112,7 +112,7 @@ hGetSomeString h sz = do
 	peekbytes :: Int -> Ptr Word8 -> IO [Word8]
 	peekbytes len buf = mapM (peekElemOff buf) [0..pred len]
 
-{- Reaps any zombie git processes. 
+{- Reaps any zombie processes that may be hanging around.
  -
  - Warning: Not thread safe. Anything that was expecting to wait
  - on a process and get back an exit status is going to be confused